The present invention generally relates to a system acquisition function. More specifically, the present invention relates to a method and system for implementing a system acquisition function for use with a communication device.
In CDMA communication systems, each base station differentiates amongst one another by using a unique PN code. A communication device, such as a mobile phone, is equipped with a system acquisition function, typically embodied in a searcher, to search for and locate the PN codes of the base stations within the vicinity of the mobile phone. Upon power-on, one of the initial tasks of the mobile phone is to find the strongest pilot signal from the nearby base stations as soon as possible. The task of finding the strongest pilot signal is commonly known as system or pilot acquisition and is usually performed by a searcher within the mobile phone.
Under one conventional approach, the system acquisition function within the mobile phone is implemented in the form of the searcher using a serial search technique that only utilizes a set of complex correlators to search for the correlation peak from one PN code offset to another. This approach consumes less power and requires less hardware; however, the search for the correlation peak may take longer.
Under another conventional approach, the searcher within the mobile phone is implemented using a traditional parallel search technique that utilizes several sets of fixed, dedicated correlators to compute the correlation peak in a concurrent manner. This other approach may shorten the search time but it does so at cost of incurring more hardware and power consumption. Furthermore, since the acquisition mode is typically less active than other modes, the exclusive use of fixed, dedicated correlators often results in a waste of hardware resources within the mobile phone.
More specifically, system or pilot acquisition in a CDMA communication system is typically performed as follows. Each base station continually broadcasts its own unique PN code in a periodic manner. One PN code from one base station differs from another PN code from another base station by an offset. Before a PN code can be identified by the mobile phone, the mobile phone first searches for signals at a particular frequency. As a result, only signals from base stations transmitting at that particular frequency are received by the mobile phone.
Next, the PN code of the base station which transmits the strongest pilot signal is identified and synchronized. The mobile phone receives signals from different base stations and these received signals are added up. Typically, the received signals are stored by the mobile phone before the correlation process begins. The mobile phone has a local PN sequence generator which is capable of generating sequences of PN codes. Initially, before the PN code of the base station which transmits the strongest pilot signal is identified, the PN sequence generator generates an initial PN code. This initial PN code is correlated with the received signals by a correlator residing in the mobile phone. Correlation is done to determine the power level of the received signals. The correlation results are examined to determine if the received signals representing the PN code of the transmitting base station fall within an acceptable time delay from the initial PN code to qualify as the strongest pilot signal. If the correlation results are below a predetermined threshold, i.e., the initial PN code generated by the local PN sequence generator does not qualify as the strongest pilot signal, then the local PN sequence generator shifts by one chip to generate another PN code and this other PN code is correlated with the received signals. The generation of PN codes and the correlation of these codes with the received signals continue until the strongest pilot signal is identified.
When the strongest pilot signal is identified, the PN code generated by the PN sequence generator and used to identify the strongest pilot signal is synchronized with the PN code of the base station which transmits the strongest pilot signal. Once the synchronization of the PN code is achieved, the mobile phone is able to communicate with the base station.
Furthermore, after pilot acquisition is completed, the mobile phone continues searching for nearby strong pilot signals and maintains a list to keep track of such signals. This process is commonly called set maintenance. That is, in addition to the strongest pilot signal, the mobile phone also searches for and keeps track of a number of additional pilot signals (and their associated PN codes) with different levels of signal strength. For example, the mobile phone may maintain an active set which keeps track of additional multipaths associated with the pilot signal of the base station that the mobile phone is currently communicating with, a candidate set with pilot signals whose strengths exceed certain threshold, and a neighbor set that includes pilot signals from cells that are in the vicinity of the cells that the mobile phone is communicating with. Maintaining a number of additional pilot signals (and their associated PN codes) facilitates the handoff process. A handoff typically occurs when a mobile phone is roaming from one area to another. This happens when a pilot signal transmitted from another base station is stronger than the one that the mobile phone is currently communicating with. The candidate set may be used to more efficiently identify the new base station transmitting the strongest pilot signal. This is because the strongest pilot signal is more likely to be one of the signals included in the candidate set. Hence, the associated PN code can be retrieved more quickly and communication with the new base station likewise can be established in a shorter period of time.
As can be seen above, the received signals need to be stored by the mobile phone so they can be subsequently used for correlation purposes. Furthermore, generation of the PN codes by the PN sequence generator is done in a sequential manner by shifting the current PN code.
Hence, it would be desirable to provide a method and system to implement a searcher for use with a mobile phone to more efficiently identify the PN code of the base station which transmits the strongest pilot signal.